The present invention relates in general to the field of electrical power distribution systems. More particularly, the present invention relates to reclosers.
A common problem in almost any electrical power distribution system is a momentary disruption of electrical service, such as might be caused by a momentary short circuit. For example, power lines strung between poles could swing under wind loading, momentary touching each other or a grounded conductor. Things may fall across exposed wires, arcing could occur, or other transitory events could cause momentary power line short circuits or current surges which could burn out a fuse or trip a circuit breaker. Most of these faults are self correcting and do not require permanent fuse or circuit breaker protection because they terminate quickly. If a fuse should burn out or a circuit breaker should trip, the power line would be open and customers would be deprived of their electrical power. Service calls to replace fuses or reset circuit breakers would then be required, thus escalating the customer""s costs.
A recloser is a fault-interrupting device used to sense current, voltage, and/or frequency and isolate faulted portions of distribution feeders. A recloser is operated by a recloser control device, which can be an electronic control circuit. Reclosers are inserted into power lines to protect a power distribution system.
A recloser senses the peak value of the current conducted and interrupting its flow by opening or tripping a recloser before a fuse blows. After an interval, the recloser closes, thereby restoring power to the system where it remains closed until the next fault is sensed.
The rate at which a fuse will blow and interrupt current is a function of the thermal heating of the fusible element. The rate of thermal heating is proportional to the power generated by the fault and each fuse has a time current characteristic which describes the time interval required to interrupt the fault current. The time interval is generally approximately inversely proportional to the value of the root mean square of the fault current. It is desirable to coordinate the recloser with the fuses to be saved to insure that the recloser in fact interrupts temporary fault currents before the fuses to be protected are blown. This is generally done by approximating the root mean square value of the fault current by sensing its peak value.
It must also be recognized that some faults which occur on a power distribution line are temporary, such as those caused by a branch momentarily falling against the line. Some faults are of a more permanent nature such as those caused by a line falling to the ground. As a consequence, reclosers are programmed so that they will trip a limited number of times within a short duration before locking open.
At some magnitude of fault current it is desirable to have the recloser open immediately to protect the line rather than following a time current characteristic. At intermediate fault levels it may be desirable from the power distribution standpoint to delay opening the recloser to allow the fault to clear itself or a fuse to operate. Many reclosers have alternate time current characteristics which achieve this goal. Typically a recloser will allow two shots or trip operations to follow a fast time current characteristic and two additional shots along a somewhat slower time current characteristic before locking open or out.
Conventional reclosers for three-phase systems open all three phases at the same time upon detection of a fault on any one phase. Other three-phase systems implement three single-phase reclosers, one on each phase. In these systems, each single-phase recloser is independently controlled. This is costly and does not allow for the phases to be responsive to one another.
Thus, in a typical configuration, for a fault, the recloser will open to clear the fault. Note that for any fault, e.g. single-phase-to-ground faults, phase-to-phase faults, phase-to-phase-ground faults, and three-phase faults, a typical recloser will open all three phases. For single-phase feeders, opening all three phases for single-phase fault results in more customers losing power than necessary. However, if an electric utility were to employ traditional single-phase protection on their distribution system, it would be implemented with three single-phase mechanical reclosersxe2x80x94one for each phase. This provides per phase approach to single-phase faults but if there is an evolving fault, such as, for example, two or more phases are faulted, then the tripping and subsequent reclosing is also always done single-phase. Totally independent single-phase devices can also be involved in a race condition. For phase-to-phase faults, if one phase was to operate more quickly than the other, the recloser may not correctly isolate the second faulted phase.
It is thus desirable to provide recloser control that can overcome the problems of the prior art. In this way, an electric utility can protect for both three-phase and single-phase faults using the same recloser.
The present invention is directed to recloser operation that is responsive to the prevailing fault conditions. A one or all phases (OOAP) technique trips a recloser for single-phase only fault conditions, or all phases for multi-phase fault conditions.
According to one embodiment within the scope of the present invention, a method for controlling a recloser for a three-phase electrical power line comprises monitoring the three phases of the power line, determining a number of faults in the three phases, and opening a number of the phases on the power line responsive to the determined number of faults.
According to aspects of the invention, determining a number of faults in the three phases comprises comparing a line current through each of the three phases to a predetermined current. A fault occurs on one of the phases when the line current on the associated phase exceeds the predetermined current.
According to further aspects of the invention, determining a number of faults in the three phases comprises detecting a fault in at least one of the three phases, starting a countdown timer associated with each faulted phase, and determining the number of faults still present after one of the associated countdown timers has expired.
According to further aspects of the invention, opening a number of phases responsive to the determined number of faults comprises opening only one phase if only one fault is determined, the only one phase being associated with the fault, and opening all three phases if more than one fault is determined.
According to another embodiment within the scope of the invention, a recloser control system for a three-phase electrical power line comprises a recloser having three poles, each pole corresponding to an associated one of the three phases of the power line and being capable of opening or closing the associated phase of the power line, and a recloser controller coupled to the recloser for controlling the setting of the poles depending on the line current appearing in the phases on the power line. One pole opens the associated phase of the power line if the line current on only the associated phase exceeds an associated predetermined current, and all poles open all the associated phases of the power line if the line current on more than one phase exceeds the associated predetermined current.
According to aspects of the invention, the recloser controller comprises a countdown timer for determining whether a phase that has entered a pickup state should be tripped.
The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.